Adjustable mechanisms are commonly used in seating where active or passive control over the chair parameters is important. Applications include office chairs, airline seating, automotive seating, lounge chairs, chairs for back pain alleviation, specialist healthcare seating for frail elderly and disabled people, and wheelchairs. The ability to alter the orientation of a chair's supports gives control over posture, muscle activity and the distribution of load within the body. The distribution of load, particularly within the upper body, is an important factor in determining the extent to which spinal structures and innervated tissues are stressed and, in long-term sitting, this may affect comfort, discomfort and pain levels. The distribution of load at the body/support interface influences compressive forces acting on the skin and muscle and is, therefore, an important consideration in comfort where blood perfusion may be occluded. For those at risk this is an important component of pressure ulcer management. Muscle activity is also an important factor in sitting, where reducing static muscle activity to a minimum has long been a fundamental ergonomic principle. As with other biomechanical phenomena, muscle recruitment is affected by body orientation and load.
The ability to alter the orientation of a chair's supports is, therefore, an important aspect in seating design. The ease with which the alterations can be made is also very important. Ergonomists argue that there is no single optimum seating posture and that the aim should he for continuous movement where “the best posture is the next posture”. This philosophy has had an important role in the development of office seating, but probably the best example of seating which achieves high comfort levels through ease of movement is the traditional rocking chair. So, there is a need for seating to do two things: to achieve biomechanically important postures and to control the ease of transition between them, whether passive or active.
A chair that aims to improve seating biomechanics has been disclosed in U.S. Pat. No. 4,790,599 (hereinafter referred to as “Goldman”). Conventional reclining chairs typically have a mechanism that recline the backrest with respect to the seat. Many also elevate or extend a leg rest either as a function of the backrest actuation or independently. In Goldman, the back section, seat section and leg rest section have a fixed structural relationship to each other (as shown in the present FIG. 1). The resulting reclinable seat structure swings inside a support structure (an outer base frame) via a seat recline mechanism; a pendulum arm connecting the seat to a swing pivot located at the approximate level of the armrests (as shown in the present FIG. 2). With this configuration and in the terminal recline position, an occupant has their feet raised above the heart level which is believed to be a more optimum position for achieving relaxation than those allowed by conventional reclining chairs.
A development from Goldman is disclosed in U.S. Pat. No. 6,012,774 (hereinafter is referred to as “Potter”), as shown in the present FIG. 3. The principal development concerns the types of design that can be used to construct the chair. In Potter it is argued that the pendulum arm connecting the seat to the swing pivot in Goldman constrains the types of design that can be realised because the pendulum arm cannot be obstructed. In Potter the seat recline mechanism involves a guide rail that is formed to follow a circumference that is defined by the pivot location in Goldman. In this way the pendulum arm is eliminated.
In both Goldman and Potter, whether physical or virtual, the seat recline mechanism has a single fixed centre rotation that defines the movement of the reclinable seat structure. This has limitations as described in European Patent No. 0 918 480 B1 (hereinafter referred to as “Samson”). In Samson it is argued that the problem with such an arrangement is the tendency of the reclinable seat structure, at least when occupied, to fall into either the upright or the fully reclined position (as shown in the present FIG. 4). This is because the combined centre of mass of the occupant and the reclinable seat structure is lower in these positions than the intermediate position requiring effort to move out of these terminal positions. In FIG. 4 this is illustrated by a circle centred on the virtual pivot point defined by the guide rail, the circumference of which passes through the centre of mass, and thus represents the motion path for the centre of mass. In Samson, the reclinable seat structure is suspended from the support structure by a pair of swing links that form the seat recline mechanism, as shown in the present FIG. 5. It is claimed that the geometry of the swing links suspending the reclinable seat structure is such that the combined centre of mass of the reclinable seat structure and any occupant remains at a substantially constant height during the movement of the chair.
A limitation in Samson is that the swing links constrain the types of design that can be used to construct the chair. This is because the swing links pivotally connect from the top of the support structure (just below the armrests) to a pendulum arm arising from the seat structure, all of which must be not be obstructed. To avoid risk of entrapment and meet the relevant safety standards, it is likely that at least the swing linkages must be concealed within a relatively large and immobile armrest, and this may inhibit ingress and egress from the side of the chair. This may be important as a fixed leg rest makes it difficult to ingress and egress from the front. Another limitation to Samson is that the use of swing linkages constrain the geometry of the seat recline mechanism. Samson will always follow two arcs defined by the swing linkages which may not be an optimal solution.
It can be seen from the prior art reported here that efforts have been made to improve the biomechanics of recline postures (Goldman), to improve the types of design that can be realised for these postures (Potter), and to improve the ease of transition between these postures (Samson). To advance beyond the prior art, there is a desire for a seat recline mechanism that delivers the same (or similar) seat recline postures with improved ease of transition, whilst allowing flexibility in respect of the types of design that can be realised.